In this work, the effects of a post-gate annealing (PGA) treatment on the electrical performance of AlGaN/AlN/GaN nanochannel high electron mobility transistors (NC-HEMTs) were analyzed, with various channel widths of 200, 400, 600, and 800 nm for a constant fill factor of 0.45. A systematic improvement in the DC parameters was observed in the NC-HEMTs after PGA treatment at 400 • C for 10 min. Secondary ion mass spectroscopy was performed on the 300 • C, 400 • C, and 500 • C for 10 min annealed and as-deposited NC-HEMT to optimize the PGA conditions. It was also verified that annealing at higher temperatures (>400 • C) can cause the diffusion of the gate metal (Ni/Au) into the AlGaN/AlN/GaN active layer, which subsequently degrades the device performance. The removal of shallow traps after the PGA treatment, which were created by ICP dry etching, improved the Schottky barrier height (∅ B ) from 0.42 eV to 1.40 eV and resulted in a significant reduction in the reverse gate leakage current (I G ) of approximately more than three orders of magnitude in the NC-HEMT with a channel width of 200 nm. The reduction in the channel resistance after the PGA treatment, correspondingly improved the drift velocity, resulting in a marked improvement in the maximum transconductance (G MMAX ) of 34% and considerable incremental increases in the maximum drain current (I DMAX ). The NC-HEMT (W NC = 200 nm) with PGA treatment exhibited decent performance, with an I G of 9 × 10 −9 A mm −1 , an I DMAX of 470 mA mm −1 , a G MMAX of 140 mS mm −1 , and an ON/OFF ratio (I ON /I OFF ) of approximately 1.1 × 10 7 along with improved gate controllability, i.e. lowering of the subthreshold swing to 69 mV dec −1 .